STF behaviour in optimised for ELD cascaded CT Delta-Sigma Modulators

Abstract

This paper looks at the Signal Transfer Function (STF) behaviour of cascaded Continuous-Time Delta-Sigma Modulators (CTDSMs) and how the flattening technique proposed by De Maeyer et al. can be used to reduce the effect of interfering signals and performance degradation for a subset of Excess Loop Delays (ELDs). Furthermore it will be shown that the noise-shaping performance of a CTDSM can be improved by optimising to take advantage of the ELD induced loop-filter order increase at the expense of the Anti-Alias Filter (AAF) response. Example cascades for two repeated second-order sections are presented which show that STF flattening and ELD optimisation offer improved performance over that of the undelayed system.